So are there any websites that grow bitcoins without risk ?
Without risk? No, I don't think there is way to multiple money without risk, even going to work is a risk - you can have an accident, you can damage some equipment etc.
Investing in casino is safer than ponzi, but earnings are much, much lower.
And as for ponzis, new ponzis appear everyday in
Economy > Marketplace > Gambling > Investor-based games subforum, if you choose right one and invest early you can make some money, but every ponzi will collapse someday and some ponzis are outright scams.